1. Field of the Invention
The present invention relates to a method and system for adaptively compensating channel or system variations in order to improve performance of precoded communications systems.
2. Related Art
Conventional systems transmit data signals at a given rate from a data transmitter to a data receiver through a channel of a transmission facility. Typically, the transmission facility has a plurality of channels. Higher data transmission rates that enable enhanced telecommunications services unfortunately give rise to intersymbol interference (ISI) when the channel frequency response is non-flat over the bandwidth of the transmitted signal. ISI results from various dispersion effects and/or multipate in the channel used for transmission which broaden pulses transmitted over the channel and causes them to interfere with one another. Unless compensatory measures are taken, the effects of ISI severely limit achievable bit error rates.
Linear equalization (LE) and decision feedback equalization (DFE) are two conventional techniques for mitigating ISI. DFE has been shown to outperform LE since DFE makes use of a feedforward filter (FFF) and a feedback filter (FBF) in the receiver, while LE only uses a FFF, see, J. E. Smee and N. C. Beaulieu, xe2x80x9cError-rate Evaluation of Linear Equalization and Decision Feedback Equalization with Error Propagation,xe2x80x9d IEEE Trans. on Commun., vol. 46, no. 5, pp. 656-665, May 1998. The typical DFE includes a feedforward filter between the channel and a first input to an adder, and a feedback filter between the output of a slicer and a second input to the adder. The output of the adder is the input to the slicer.
The performance of DFE can deteriorate due to the effects of error propagation, in which detection error advances through the FBF. To achieve the performance of a DFE without error propagation, a method using a precoder was proposed in which a FBF is implemented at the transmitter, instead of the receiver, as described in M. Tomlinson, xe2x80x9cNew Automatic Equalizer Employing Modulo Arithmetic,xe2x80x9d Electron Lett., vol. 7, pp. 214-218, March 1971 and H. Harishima and H. Miyakawa, xe2x80x9cMatched-Transmission Technique for Channels with Intersymbol Interferencexe2x80x9d, IEEE Transactions On Communications, Vol. Com-20, No. 4, pp. 774-780, August 1972. This method precompensates for postcursor ISI without increasing transmitted power. Modulo arithmetic is used to bound the transmitted power. FIG. 1 illustrates a conventional Tomlinson Harashima precoding system. Precoder 102 provides precoder coefficients as input to module 104 which reduces b[n] to y[n]. Feedforward filter 105 receives output from communication channel 106. Module determiner 107 reduces output of feedforward filter 105 which is applied to slicer 108. Conventional methods typically use a combination of a feedback filter (FBF) with the feedforward filter (FFF) in the receiver during training of the precoder. When the training is complete, the FBF coefficients are transmitted to the precoder where an equivalent FBF is implemented. Thereafter, the FBF in the receiver is disconnected. In order to compensate for changing channel conditions the FFF in the receiver is adaptively adjusted by a decision-directed algorithm with linear compensation.
Precoding has found widespread use in voice-band modems where precoding can be effectively combined with trellis-coded modulation to enable bit rates of 33.6 kb/s to 56 kb/s in a bandwidth of less than 4 KHz, as described in G. D. Forney Jr. and M. V. Eyuboglu, xe2x80x9cCombined Equalization and Coding Using Precoding,xe2x80x9d IEEE Commun. Mag., pp. 25-34, December 1991 and M. V. Eyuboglu and G. D. Forney, xe2x80x9cTrellis Precoding: Combined Coding, Precoding and Shaping for Intersymbol Interference Channels,xe2x80x9d IEEE Trans. Inform. Theory, vol. 38, no. 2, pp. 301-314, March 1992.
A conventional voice band modem can connect computer users end-to-end through the Public Switched Telephone Network (PSTN). However, the twisted pair telephone subscriber loop of a computer user has a much wider usable bandwidth. The term digital subscriber line (DSL) has been used to refer to technologies which offer significantly higher bit rates, from 1.5 Mb/s to 52 Mb/s, over the local loop twisted-wire-pairs which connect the service provider""s Central Office to a customer""s premises, as described in J. M. Cioffi, V. Oksman, J. J. Werner, T. Pollet, M. P. Spruyt, J. S. Chow, and K. S. Jacobsen, xe2x80x9cVery-high-speed Digital Subscriber Lines,xe2x80x9d IEEE Commun. Mag., pp. 334-343, March 1997. With advancing technology and an increasing desire for higher downstream bit rates, standardization efforts have resulted in a series of DSL embodiments collectively referred to as xDSL with the acronyms high-speed (H), asymmetric (A), and very high-speed (V); for example, HDSL, HDSL2, ADSL, and VDSL.
U.S. Pat. No. 5,987,061 describes a modem that operates in the voice-band frequency and the xDSL frequency bands. The modem uses a Digital Signal Processor (DSP), so that different existing ADSL line codes, such as Discrete MultiTone (DMT) and Carrierless AM/PM (CAP), can be implemented on the same hardware platform. The modem negotiates, in real-time, for a desired line transmission rate to accommodate line condition and service-cost requirement. The line code and rate negotiation process can be implemented at the beginning of each communication session through the exchange of tones between the modems. A four-step mid-band digital subscriber line (MDSL) modem initialization process is provided for line code and rate compatibility. The system includes a direct equalizer system with an adaptive filter in the transmitter for symmetrical dispersive transmission channels. The filter coefficients are identified in the receiving path using shift and addition operations. A training sequence is multipled with data at the transmission path. In the receiver path, a received data detection function controls adaptation of transmitter filter coefficients. The combination of the transmit filter and its adaptation mechanism forms the direct channel equalization. The filter coefficients are updated periodically using a (DSP) in a few baud intervals.
Provided that the fading is slow enough, preceding of the FBF can also be applied to wireless channels with ISI. In wireless and other environments, it is often desirable to use multiple receiver antennas and multiple feedforward filters. It has been suggested to implement both the FFF and FBF operations of the DFE at the transmitter to transfer complexity from the wireless user to the basestation, as described in A. B. Sesay and M. R. Gibbard, xe2x80x9cAsymmetric Signal Processing for Indoor Wireless LANs,xe2x80x9d in Proc. of the IEEE 6th Int. Symp. on Personal, Indoor, and Mobile Radio Commun., PIMRC 95, Toronto, Canada, Sep. 27-29, 1995, pp. 6-10. This approach has the limitation that problems arise in maintaining a peak power constraint.
An adaptive LE in the receiver has been used to deal with precursor ISI, as described in P. R. Chevillat and E. Eleftheriou, xe2x80x9cDecoding of Trellis-encoded Signals in the Presence of Intersymbol Interference and Noise,xe2x80x9d IEEE Trans. on Commun., vol. 37, no. 7, pp. 669-676, July 1989. In this implementation a whitened matched filter (WMF) precedes an augmented maximum likelihood sequence detection (MLSD) which is assigned the task of trellis decoding in the presence of postcursor ISI. The motivation for making the WMF adaptive stems from the desire to compensate for slowly varying channels. Adaptive linear techniques have more recently been applied to the DSL, as described in S. McCaslin and N. van Bavel, xe2x80x9cHDSL2 Performance with Run-Time Precoder Coefficient Updates,xe2x80x9d Proposal to Standards Committee of T1-Telecommunications, Subcommittee T1E1.4, study group in Huntsville, Ala., Jun. 1-4, 1998 and E. Shusterman, xe2x80x9cPerformance Implications of a Non-additive Tomlinson-Harashima Precoder,xe2x80x9d Proposal to Standards Committee of T1-Telecommunications Subcommittee T1E1.4, study group in Austin, Tex., Mar. 2-5, 1998.
U.S. Pat. No. 5,513,216 describes the use of a hybrid DFE in which a precoder and modulo device is used at the transmitter while an FFF, a modulo device, a noise-predictive (NP) FBF, and an ISI canceling FBF are used at the receiver. The ""216 patent has the shortcomings that the slicer output serves as the input to both the ISI canceling FBF and the NP FBF, thus leading to a less reliable estimate of the transmitted symbol which is applied to the receiver modulo operation.
It is desirable to provide an improved method for adaptively compensating for channel or system variations.
The present invention relates to a method and apparatus for adaptively compensating for channel or system variations in which adaptive compensation is used in the receiver of a digital communication system. The transmitter of the digital communication system includes precoding. The adaptive receiver compensation mitigates the interferences not removed by the transmitter precoder.
In an embodiment of the invention, the adaptive compensation can be performed using an adaptive feedforward filter (FFF) and a feedback filter (FBF) in the receiver. The FBF output is generated based on previous values of estimates of the transmitted precoded sequence. The determined value of the FBF coefficients can be periodically relayed to the transmitter to update the precoder coefficients of the transmitter. Alternatively, the value of the FBF coefficients can be relayed to the transmitter after the value of the coefficients exceeds a predetermined threshold. Accordingly, the receiver adaptively and automatically compensates for misadjustments of the fixed transmitter precoder with respect to the actual channel at a given point in time.
In the present invention, the output of the receiver""s FFF is added to the output of the receiver""s FBF, and the sum is applied to the input of the receiver modulo device. The input to the postcursor ISI canceling FBF is not the slicer output itself, but is instead a version of the slicer output which is modified to account for the receiver modulo device. The receiver does not include the use of a noise-predictive FBF. The approach offers improved performance since a more reliable (with both precursor ISI and postcursor ISI accounted for) estimate of the transmitted symbol is applied to the receiver modulo operation, thus making it more likely that the receiver modulo correctly cancels the action of the transmitter modulo operation for that symbol.
The apparatus and method can also be used in conventional narrowband and wideband spread spectrum communication systems including digital subscriber line (DSL) modems having high data rates such as HDSL and VDSL and direct-sequence spread spectrum systems, such as DS-CDMA.
The invention will be more fully described by reference to the following drawings.